There are use flags for assembly optimizations. Your CPU has the following.

USE="3dnow 3dnowext mmx mmxext sse sse2 sse3"

Do the sse keys act recursively? If I add sse3 to my USE, will sse and sse2 automatically be applied as necessary? I seem to remember that being the case for CFLAGS, but am unsure about USE._________________This space for rent... Well maybe to give away.. Heck.. i'll pay you to take it.

"cat /proc/cpuinfo" should tell you what the cpu has available in its
instruction set (see the "flags" entry). "-march=native" in CFLAGS
in /etc/make.conf should pull in any of those that are relevant to
the current code that gcc is compiling._________________TIA

Doesn't this mean that all of these are automatically enabled for all amd64 profiles?

It would simply mean that they are not forbidden (masked). Whether they are actually
used by a particular ebuild would likely be conditional on other tests (since not all
amd64 cpus implement all of those instruction set extensions)._________________TIA